Pioneer SX-780 receiver

[MikeS]

Picked up this pig off Craigslist for my buddy Jeff.
(It's more minty than the pic shows, flash usually pulls out dust and grease marks.)

I bought a Harmon Kardon 330B for Jeff 4-5 years ago. A rather nice little 17wpc receiver from 1974 or so.
It still works, but the pre-driver stage has something fucked up. Going main amp in direct, and it works fine.

Anyways, he brought it over...it's fried...wah...I need another receiver......
He prefers a vintage unit, and with nothing but 5.1 AV trash pretty much out there, I don't blame him...and he's on a restricted budget as well.

So I lurk the local Craigslist......nothing much going.....some asshole around has been glomming on to every vintage squalid scrape unit around here.
Missed out on a nice vintage silver-faced JVC tuner and integrated amp, cassette deck and speakers thrown in for 40 bucks.
Listed local here in Cheney...I'm sure that douchebag dove on that......

Spotted the listing for the Pioneer SX-780. The dude had just listed it the evening.
It was a email reply only listing. I think my ace in the hole was I emailed him at about 3:00AM.
And that douchebag dude did call him, BTW....

The receiver was owned by one of the head dogs at KREM 2, a local TV station.
He bought it in 1979 for 266 bucks, still has the original box....
He took good care of it.

These were like lower midline receivers in the day, they use those STK output module transistors, (power paks?).
45wpc, and it seems a rather conservative rating.

The thing has plenty of power, way way overkill for my 104db Lascalas.
I can't get those meters past a watt, or it's mega loud......

It's not bad sounding with the LaScalas when not pushed much over a watt.
Strong tight bass, rather open sounding for a SS unit, and rather nuetral/transparent sounding as well.
It images pretty well, actually.

A wee bit bright, but that's kinda the Pioneer sound. In my old age, I kinda apreciate the extra ommph on top. And can adjust the treble pot if need be.

The tuner section works very, very well. Radio staions sounds like Hi-Fi, the local Classical station sounds glorious.

I'll give it the once over this weekend, and then he should be good to go.....

[todd.brust]

nice pick up there...looks really close to my kenwood kr-6050...i am very happy with mine also...it is just about mint...is the tuner on that one the kind that you spin it and it keeps spinning, like it is weighted or something? and i love the analog dials...too bad no one does that anymore these days unless you get into th really high end stuff :(

how much did you pick that up for?

[X-OvrDistortion]

That looks just like the one I had when I was a kid. I wish I kept that thing.

[MikeS]

I paid too much.....75 bucks. The guy wouldn't haggle either......oh well.

The way I look at is you'd be hard pressed to find new 2 channel receiver for 75 bucks. One thing, it's all multichannel AV receivers pretty much.

And I think the starting rate is $100 and above for a AV receiver. You might find one with 4 channels at 50 watts.

But I can bet the old Pioneer's 45 wpc is helluva lot more ballsy sounding than a AV receiver's rated 50 watts.

I dunno, there is some lower wattage AV receivers that sound rather good from what I understand. But they start around 200 bucks.

I guess I fell for the looks of the unit. Beats the black box look, IMO.

I'm sure Jeff will like it....I hope so, he already paid for it.

He's got some pretty decent oak Pioneer tower speakers from the 1980's, with a six disc Pioneer changer. So he'll have a full blown pioneer rig.

He just got around a 37 inch or better LCD television. He can feed the DVD and cable box right into the receiver. He ain't gonna give a fuck about multichannel...

So I get this lowly little 17wpc Harmon Kardon 330B receiver in return.

This old receiver sounds just a little dull and flat. which i think is the reason folks like these old Harmon Kardon units with Klipsch speakers.
My theory: The electrolytic capacitors are 30+ years old and dried up, pretty much turned into resistors.
The flat dullish sound kinda helps with the high efficiency Klipsch to keep them from ripping your face off......

The Pioneer doesn't sound dull or flat. Bright and a little shouty with Klipsch, but not dull and flat.

If I can troubleshoot the problem with the Karmon Hardon and fix it, I'll go ahead and re-cap the unit. The little HK 330B does still sound good, and a recap should improve it more.
It has it's wood case... a real laminated wood case...

With the left and right mono tube pre-amps, the little HK sounds a little bit better yet.

[MikeS]

Ok....took a mascara brush and pulled the cover. Brushed 29 years of dust/dirt off the tuner board. The rest of the boards weren't too bad.

The receiver had a scratchy volume pot at low levels, and the tuner section was getting a scratchy noise while playing stations.

After a good half of brushing, I then blew everything down with a can of compressed air. I hit that tuner board/tuning cap a few times....

A search on the AK site revealed many tips with regards to SX-780.
There are three power supply regulators, with sinks. One is right near the main power paks heat sink.

The regulator does get warmer than the others. I dunno if it's a big concern, but they mention the solder pads will get dry/cold or burnt looking, and should be touched-up. Sure as shit they were dry and cold looking.
I touched up all the regulators solder pads.

I then hit that volume control with cleaner. Douched it hard, frequently turning.
Let it sit, blew the shit out of it with compressed air.
I let it dry for about 15 minutes, then hit it again with the cleaner. Blew it out again, and let it dry.

While it dried, I went ahead and cleaned the sides and top panel along with the bottom panel. Knobs and faceplate as well.
When the pot was dry, I fired it up and checked DC offset.

There is no bias adjustment for these goofy-assed Darlington Power Pak 0050 power modules. A gloried power IC? A power slop amp of some sort?
They spec for 50 watts.
I think a lot of the late '70's to 80's SS receivers started using these power pak module things. I have a JVC integrated that uses a variant of this module for outputs.

They just have DC offset, and meter calibration. Offset measured around 8-10 millivolts for the right side, -31 millivolts for the left side.
They want you to try to adjust bias for 0 volts. I did my best with these frikkin touchy trim pots and managed 4-8 millivolts for each side.
After four hours, the bias remained at 4-8 millivolts between sides.....that's good I guess.

The receiver sounds really good now. No scratchy shit with the pots, no tuner noise, all is well for now.....

Here's the bad boy receiver for 1978. Makes this 1979 SX-780 look like a serious POS....

http://www.silverpioneer.netfirms.com/sx-1980.htm

I guess one can still get some serious quality SS...provided you live in Japan and are rich....nice amp that M8.

http://www.silverpioneer.netfirms.com/exclusive_amps.htm

[todd.brust]

looks great, i love the look of the 70s SS stuff. just out of curiosity, what do you use to clean the volume pots with?

[MikeS]

Some shit from Radio Shack...tuner and pot cleaner...works for me....

I think I may have found the problem with this Harmon Kardon 330B.
Checking voltage from PS to all the PCB's. This first cap after the main rectifier diodes shows 52 volts DC.

I'm getting 54 volts, ok...that ain't bad. Then from the main filter cap, the 54 volts runs to another section with a 1000 uf cap, and 2200 uf cap, a diode, and a couple resistors.

This is the filtering/voltage dropping section for the pre-amp and pre-driver boards. I should have take off voltages off two points from this PCB of 17 and 19 volts.
I get 1.0 and 1.5 volts from these two points. A little inspection, and the diode looks pretty much cooked, along with a carbon comp resistor.

IIRC, a fried diode will just do forward voltage drop pretty much both ways....does that sound right?

I've got caps to replace with, and bought some diodes from RS.
Hopefully I have the value in resistors to replace these.

It may live as a receiver again.....

[MikeS]

That worked.....I have a little more voltage than I should, and may have to drop it a bit. But it boogies again.